Add duo_chat_clarification_question_tool feature flag

What does this MR do and why?

Add gitlab_com_derisk feature flag duo_chat_clarification_question_tool to control the enablement of the clarification_question tool in Duo Workflow Service.

References

• Implement clarification_question tool for Duo C... (#600650)
• [FF] `duo_chat_clarification_question_tool` -- ... (#600653)

Screenshots or screen recordings

Not applicable - feature flag definition only.

How to set up and validate locally

Not applicable - feature flag definition only.

MR acceptance checklist

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